The long-term goal of this research is the elucidation of the mechanisms of protective immunity against Ehrlichia chaffeensis, the causative agent of human monocytotropic ehrlichiosis (HME). Achievement of this goal requires knowledge of which humoral and cellular immune mechanisms stimulated by ehrlichiae are effective in the clearance of ehrlichiae. HME is a life-threatening tick-borne infection associated with adult respiratory distress syndrome, meningitis, and shock in immunocompetent patients, overwhelming infection in immunocompromised patients, and a fatality rate of 2.7 percent. More than 2,200 cases have been diagnosed with laboratory confirmation, and the incidence is 1,000 cases per 1,000,000 population in tick-exposed rural populations. The specific aims test the hypothesis that the immunodominant, surface-exposed p28 antigens stimulate protective immunity by antibodies and cellular mechanisms and determine the importance and mechanisms) of antibodies and cellular mechanisms of protective immunity in mouse models of HME. The research design includes sequencing of the loci of the p28 multigene families of major immunodominant surface proteins of Ehrlichia muris and a related ehrlichia (IOE) DNA and recombinant protein vaccines in the E. muris mouse model and the highly pathogenic IOE-C57BL/6 mouse model. Humoral immunity will be passive polyclonal and monoclonal antibodies to the conserved and variable regions of p28 families in IOE-infected mice including Fc-receptor knockout mice. Opsonization will be investigated in murine macrophages in vitro with E. muris and specific polyclonal and monoclonal antibodies. Cellular immune mechanisms will be elucidated using gene knockout mice (MHC Class I, MHC Class II, delta T-cell receptor, IFN-gamma, perforin, iNOS, and TNF-alpha receptor) and TNF-alpha depleted mice, immunohistochemical and flow cytometric analyses of the cell subsets and their cytokine profiles, adoptive transfer of T-lymphocyte subsets including T-cell clones in the outstanding new mouse model of HME.